EIA: Transmissible Through the Air?

An outbreak of equine infectious anemia (EIA) at a veterinary hospital in Ireland is leading some researchers and veterinarians to postulate the virus might be transmissible through the air in some exceptional circumstances. This is the first time scientists have observed a situation that points to aerosol transmission of the virus.

In summer 2006, the virus spread from one mare to all the other adult horses sharing one barn at Troytown Equine Hospital in County Kildare, Ireland. All of those adults became ill and were euthanatized over the following five months.

"At the time, on the 14th of June, we were confident that there no biting flies present and that the spread of the disease via contaminated needle or instrument could not have occurred," said Michael Sadlier, MVB, MRCVS, CertESM, CertES, MACVSc, a partner at Troytown. "The biosecurity in Troytown Hospital meets the highest international standards. The subsequent spread of the disease to other horses in the same barn must have been by a previously undocumented means."

Until now it has been thought that the virus can be spread only via the blood on shared needles or instruments or by blood-feeding insects.

The 2006 outbreak was Ireland's first experience with EIA.

The index mare was referred to the hospital on the evening of June 12 with strange clinical signs consistent with liver failure. The mare's foal had died several months earlier of an unknown illness. Personnel placed the mare in a stall in the hospital's main barn and she showed overnight improvement following treatment for the clinical signs she was exhibiting. The following afternoon (June 13), she began to bleed passively from both nostrils. This bleeding became consistently more pronounced, and the mare was snorting heavily from both nostrils, until the treating veterinarians led her out of the barn and euthanatized her at 4 a.m. on June 14. Hospital staff removed the contaminated materials (bedding, hay, blood) from her stall, disinfected the stall and then pressure washed the barn aisle.

"On the morning of the 14th of June, after discussions with the referring veterinarian, the Irish Equine Centre (an independent organization that provides laboratory services for the diagnosis, management, and prevention of diseases in Irish horses) and the Department of Agriculture and Food (DAF), laboratory tests were performed which confirmed the diagnosis of EIA," said Sadlier, "As EIA is a notifiable disease, the control and monitoring of the disease then came under the direct control of DAF."

It was later revealed that the index mare's foal had been given plasma that was illegally imported from Italy--the owner of the mare's farm brought the plasma products into Ireland. According to Sadlier, this plasma most likely originated at slaughter facilities processing horses from Eastern Europe, where EIA is a serious problem.

In the three days following the postmortem diagnosis of the mare, none of the other horses in the hospital tested positive for EIA on standard tests. But on July 18, 35 days after the index mare bled at Troytown, four horses that had shared the barn with her that night became sick and tested positive for EIA. After those four became sick, hospital staff notified the owners of all the other horses that had been in the barn on June 13. "These horses had been discharged from the hospital previously and had been placed under restriction notices at home by DAF," Sadlier said.

The partners at Troytown Equine Hospital immediately closed the hospital for further admissions and the horses that were in the hospital at that time (a total of 37) were moved to a quarantine center. After depopulating the facility, Troytown Equine Hospital was closed for a month, and the entire facility was cleaned, disinfected, repainted and then certified by DAF as acceptable to re-admit horses.

"Two of those four (ill) horses were mares whose foals were sick--these mares had never been touched," noted Sadlier. "They had never been injected, they had no procedure done to them. And that's when we knew some unique event had occurred in our hospital. The only common factor between these mares and the index case was the fact they were stabled within the same barn, breathing the same air."

A Coggins test is used to confirm a horse's EIA status.

Of the adult horses in the barn on July 13, all would eventually test positive for EIA. All of the horses were tested every 10 days from June 14 and some of the horses did not return positive tests until 145 days after exposure.

"Initially, we suspected it was the power-hosing that aerosolized the virus," Sadlier said. However, one mare that was not in the barn at the time of the clean-up--and subsequently became positive to EIA--eliminated the theory, according to Sadlier. This mare was in the hospital because her foal was being treated for contracted tendons. She was in the affected barn for 1½ hours on the evening of June 13 (while her foal was anaesthetized and limb casts applied), before the index mare was euthanatized. She then left the barn and was stabled in another barn overnight. She was not there when the pressure washing was performed, and she was the only horse in the hospital's other barns to eventually test positive.

"Our hypothesis for aerosolization of the virus is that the index mare did this by continuous blowing through blood-filled nostrils," said Sadlier. "The epidemiologists from DAF and ourselves have looked at it every possible which way--I don't think it's anything to do with power-hosing because we had the one separate mare. What has really been a concern to everybody who has examined the facts of this outbreak is that four of the affected horses were not injected or subjected to an invasive procedure. This fact would suggest that the viral spread in the barn was not by a previously recognized route."

In total, 27 Irish horses have tested positive for EIA since June.

"Epidemiologists and scientists from Ireland, the U.K., and the Gluck Institute in Kentucky (the University of Kentucky's Gluck Equine Research Center) have accessed all our records and to date, the aerosol theory has been universally accepted," Sadlier said. "We'd like to know how it happened, how we got such a phenomenal infection rate. The irony is that from our precise hospital records we have more information about this outbreak of EIA than ever documented previously. This will be continued to be presented at scientific conferences and in research gatherings to inform veterinarians and horse owners in general in order to try and prevent this awful occurrence ever happening again."

Equine infectious anemia is an untreatable disease for horses--once infected, they remain infectious for life and in the United States either they must be euthanatized or permanently separated from other horses to prevent the spread of the disease. Equine infectious anemia is closely related to the virus that causes acquired immune deficiency syndrome (AIDS) in humans, and a horse's EIA status is confirmed using a method called the Coggins test.